This invention pertains to microlithography (projection-exposure) of a pattern, defined by a reticle or mask, onto a sensitive substrate such as a semiconductor wafer using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used in the fabrication of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods in which image defocusing and distortion caused by Coulomb effects are reduced.
As the degree of integration of semiconductor integrated circuits has increased in recent years, more intricate circuit patterns have been demanded. As the desired minimum linewidth in circuit patterns has fallen to 0.1 xcexcm and below, the inability of optical microlithography to provide acceptable resolution is apparent. Consequently, substantial research currently is being devoted to the development of a practical charged-particle-beam (CPB) microlithography apparatus that can provide the desired pattern resolution at a satisfactory throughput.
Initial efforts in this regard were directed to CPB microlithography systems that performed exposure of an entire chip pattern (xe2x80x9cdiexe2x80x9d), or even multiple dies, in one xe2x80x9cshot.xe2x80x9d Such systems currently are impractical because of the extreme difficulty of preparing a reticle that can be exposed in one shot and of maintaining aberrations below necessary levels within a CPB-optical field sufficiently large to accommodate an entire die or multiple dies.
As an alternative to xe2x80x9cone-shotxe2x80x9d CPB microlithography systems, so-called xe2x80x9cdivided-reticlexe2x80x9d CPB microlithography systems are being investigated actively. In a divided-reticle system, rather than exposing an entire die or multiple dies at one time, a die as defined on the reticle is divided into small regions (e.g., xe2x80x9csubfieldsxe2x80x9d) each measuring about several hundred xcexcm square, and the small regions are exposed sequentially in an ordered manner to effect exposure of an entire die. During exposure of each small region, correction of exposure parameters can be made as required, such as correction of focus and distortion of the image being formed on the substrate (xe2x80x9cwaferxe2x80x9d). Thus, exposure can be performed with good resolution, accuracy, and precision over an even wider area than obtainable using a one-shot system.
CPB microlithography systems are subject to certain problems such as defocusing and distortion of the image due to Coulomb effects. A Coulomb effect is caused whenever neighboring charged particles in the charged particle beam repel one another sufficiently to disturb the trajectories of the charged particles in the beam. Particles having disturbed trajectories degrade the image. Coulomb effects are also exhibited by divided-reticle systems. Current attempts to reduce Coulomb effects include adjusting focus by altering the electrical current supplied to a refocus lens, etc. However, Coulomb effects cause not only shifts in focal point but also distortion of the image as projected onto the wafer.
An example of a CPB-optical system exhibiting reduced distortion caused by Coulomb effects and that includes three focus lenses and two stigmators is disclosed in Japan Kxc3x4kai Patent Application No. Hei 11-87208 (1999). Whereas this CPB-optical system exhibits reduced lower-order distortion, higher-order distortion is not corrected satisfactorily. Also, because this system performs correction of focus and distortion for each subfield as projected, this system is complicated. Furthermore, the subfield-by-subfield corrections require substantial time for performing the necessary measurements and calculations.
According to another conventional approach to reducing Coulomb effects, distortion that otherwise would be created by Coulomb effects is estimated or measured in advance and the reticle pattern is distorted deliberately so as to achieve, when the subfields are exposed, an offsetting distortion. However, as with the other approach summarized above, the Coulomb distortion has to be calculated or measured in advance, which adds complexity and processing time.
Another conventional approach that offers prospects of overcoming certain of the lingering problems involves shaping the charged particle beam to have a ring-shaped transverse profile (i.e., a xe2x80x9chollow beamxe2x80x9d). This approach is disclosed in Japanese Kxc3x4kai Patent Application No. Hei 11-297610 (1999). For example, FIGS. 4(a)-4(b) herein depict this approach for forming a hollow beam. An annular aperture 24 is provided by defining segmented voids 25B into a plate 25A made of molybdenum or tungsten. The segmented voids 25B collectively define an essentially annular void. The axis of the annular void is coincident with the CPB axis (i.e., in the middle of the beam 21). As the beam 21 strikes the plate 25A, portions of the beam incident at a segmented void 25B are transmitted through the plate 25A; all other portions of the beam are blocked (scattered and/or absorbed) by the plate 25A. The portions 22 of the beam passing through the segmented voids 25B are used for illuminating the reticle (located downstream) and for transferring the pattern. The annular aperture 24 is disposed at a crossover plane A at which the beam is narrowly constricted.
A conventional CPB-optical system (specifically an electron-beam system) utilizing an annular aperture 24 as summarized above is shown in FIG. 5. The FIG. 5 system is essentially as disclosed in the JP 11-297610 reference cited above. An electron beam 21 emitted from an electron source 26 passes through an xe2x80x9cillumination-optical systemxe2x80x9d comprising illumination lenses 27, 28, and strikes the annular aperture 24. A field-limiting aperture 29 is used to shape the beam 21 so as to illuminate a desired shape (e.g., square) of subfield on the reticle 33. The field-limiting aperture 29 is situated at an axial location that is conjugate with the electron-emission surface of the source 26, with respect to the illumination-optical system. An image of a first crossover 30 (near the source 26) is formed by the illumination lenses 27, 28 on the annular aperture 24. I.e., the annular aperture 24 is disposed at a crossover position. The hollow beam 22 passing through the annular aperture 24 passes through a third illumination lens 32. The third illumination lens 32 forms an image of the electron-emission surface of the source 26 on the reticle 33, thereby illuminating the reticle 33. The field-limiting aperture 29 is situated at an axial position that is conjugate with the reticle 33, with respect to the lens system comprised of the illumination lenses 28, 32. An image of the illuminated portion (subfield) of the reticle 33 is formed on the wafer 36 by projection lenses 34, 35 collectively constituting an xe2x80x9cprojection-optical system.xe2x80x9d A contrast aperture 37 is situated so as to block particles of the beam that are scattered by passage through the reticle 33.
Coulomb effects are diminished using the FIG. 5 system as a result of shaping the beam 21 into a hollow beam 22 before the hollow beam 22 irradiates the reticle 33. However, manufacturing the annular aperture 24 for use in the FIG. 5 system is problematic. Namely, the diameter of the crossover at which the annular aperture 24 is placed normally is several hundred xcexcm. Whenever an annular aperture (defined by a plate several hundred xcexcm thick) is placed at such a position, the temperature of the plate can reach several thousand degrees C during use. Hence, the material used to make the annular aperture 24 is limited to high-temperature metals such as molybdenum or tungsten. It is extremely difficult to fabricate the necessary voids, having dimensions in micrometers, in molybdenum or tungsten stock that is several hundred xcexcm thick. Also, the high operating temperatures experienced in the vicinity of the annular aperture during use causes detrimental thermal instability in the CPB-optical system.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide a hollow-beam CPB microlithography apparatus in which heating of the annular aperture to high temperature during use is prevented. Another object is to provide methods for fabricating a suitable annular aperture from easily worked materials.
According to a representative embodiment, a system is provided for producing a hollow illumination beam in a CPB microlithography apparatus. The system comprises a scattering aperture several micrometers thick situated at a crossover-image plane of the illumination-optical system.
The scattering aperture can be configured as voids defined in a beam-scattering plate. The voids are arranged in a region of the plate having a first radius equal to a radius of a central region of the plate and a second radius greater than the first radius. Thus, a hollow beam is formed downstream of the scattering aperture as the beam passes through the voids. Alternatively, the scattering aperture can be configured as one or more openings in a layer of CPB-scattering material on a relatively CPB-transmissive membrane. The opening(s) can be configured similarly to the voids summarized above or can have a full-ring (donut) profile.
The system also comprises a blocking aperture situated between the scattering aperture and the reticle. The blocking aperture desirably is configured as a plate defining a central void. The plate is configured to absorb charged particles of the beam that were scattered by passage through the scattering aperture.
As noted above, the scattering aperture is made from a plate that scatters rather than absorbs charged particles. Because few charged particles are absorbed, the plate is not heated to a high temperature during use. This allows the scattering aperture to be made of a more easily micro-machined material (e.g., silicon) that does not have to withstand high temperature during use. Even though charged particles scattered by the scattering aperture are absorbed downstream by the blocking aperture, the beam at the blocking aperture is more spread out, which substantially lessens the beam-current density impinging on any given area of the blocking aperture. (For example, the beam-current density on the blocking aperture is about {fraction (1/10)} the beam-current density on the scattering aperture, which is located at a crossover.) Also, the thickness of the plate defining the blocking aperture can be several mm thick. As a result, the blocking aperture is heated during use to a temperature of only several hundred degrees C.
The lower operating temperatures of both apertures solves the above-summarized problems experienced with prior-art apparatus that include apertures that undergo heating to substantially higher temperatures during use.
The scattering aperture is placed near the crossover because the illumination-optical system normally is configured so that a relationship of beam intensity versus lateral distance from the optical axis at the crossover is similar to the relationship of beam intensity versus angle of incidence at the reticle surface. Because of this relationship, if the scattering aperture is placed at the crossover, then the charged particles of the beam will strike the reticle surface at a nearly constant angle of incidence with no loss of illumination uniformity.
xe2x80x9cNearxe2x80x9d the crossover means that the position of the scattering aperture may be separated to some extent from the crossover position. The amount of separation that is acceptable can be determined by a person of ordinary skill in the art according to the illumination uniformity demanded.
Since an object is to form a hollow beam before the beam strikes the reticle (while scattering and absorbing other parts of the original beam), it will be appreciated readily that such a beam can be produced even if the voids in the scattering aperture are, for example, segments of a polygon rather than segments of a ring.
A system according to the invention can include a current-limiting aperture situated upstream of the scattering aperture. The current-limiting aperture is configured to pre-absorb particles of the charged particle beam at an edge of the beam. With such a configuration, the thermal load on the scattering aperture is alleviated since the charged particles of the beam at the beam edges are absorbed by the current-limiting aperture before the particles enter the scattering aperture.
The system also can include a scattered-particle-absorbing aperture, configured as a particle-absorbing plate defining a central void, and configured to be situated between the blocking aperture and the reticle. Such an aperture effectively absorbs scattered charged particles of the beam that passed through the scattering aperture. If necessary, multiple scattered-particle-absorbing apertures.
According to another aspect of the invention, CPB microlithography apparatus are provided that comprise a system, as summarized above, for producing a hollow illumination beam. According to yet another aspect of the invention, methods are provided for manufacturing a semiconductor device. Such a method comprises performing projection-transfer of a pattern, defined by a reticle, onto a wafer using a charged-particle-beam microlithography apparatus as summarized above. Since Coulomb effects are diminished substantially, image defocusing and distortion can be reduced without decreasing the beam current. Hence, semiconductor devices can be manufactured with high resolution and high accuracy without decreasing throughput.
In any event, a system according to the invention effectively solves the problems inherent to the JP 11-297610 apparatus, while preserving the benefits of an apparatus according to that reference.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.